This invention relates to systems and methods for separating, modifying or combining wavelength multiplexed signals in optical communications, and more particularly to such systems and methods for controlling, modulating, switching or otherwise manipulating individual wavelength signals in dense wavelength division multiplexed beams.
New and demanding problems in optical communications constantly arise for dense wavelength division multiplexing (DWDM) based transmission at high data rates, dense channel spacings and high channel counts. These problems include combining, separating, filtering, attenuating and switching any or all of the wavelength channels in an optical fiber. There is a need for new devices that perform these functions with wide, flat passbands, low channel crosstalk, precise attenuation, and high channel extinction. In addition, these functions should be performed while maintaning low insertion loss, low polarization dependent loss, and low chromatic dispersion. As channel spacings decrease from 100 GHz down to 50 GHz and less, new challenges in achieving these requirements are constantly introduced. Furthermore, it is highly desirable that the functions be effected within a low profile and small footprint package and that the components and systems be readily manufacturable by conventional processes, so that the resulting systems are cost effective and commercially viable.
Prior art techniques for providing some, but not all of these functions use fiber Bragg gratings, thin film filters, planar waveguides and diffraction gratings. While these approaches may individually satisfy a subset of these requirements, they have limited capability for satisfying all. For example, the application of diffraction gratings to DWDM devices have been thus far limited due to passband roll-off, high insertion loss, challenges in stable fiber-to-free space coupling, and physically large size.
The use of diffractive optics in free space spectrometers, for example, is well known, as described in U.S. Pat. No. 2,922,331 to W. G. Fastie et al. (1960) and an article by Fastie et al., entitled xe2x80x9cMultiple Diffraction in Grating Spectroscopyxe2x80x9d, Journal of the Optical Society of America, Vol. 44, No. 2, February 1954. In U.S. Pat. No. 2,922,331, Fastie et al. describe a wavelength dispersive device obtained by propagating an optical beam onto a diffraction grating, forming the wavelength spectrum at the Fourier plane defined by a separate focusing mirror system, and modifying the individual beam elements comprising the emergent wavelength spectrum. The object light source, shared reflector with effective focal length f, diffraction grating, and image/analysis plane, wherein the distances between the elements are equal to f, comprise what is known in the art as a 4-f spectrometer.
Early work on spectrometric applications using diffractive optics is to be found in various references, such as xe2x80x9cSuccessive Diffractions by a Concave Gratingxe2x80x9d, Jenkins et al., Journal of the Optical Society of America, Vol. 42, No. 10, October 1952, pp. 699-705 and xe2x80x9cInfrared Grating Spectrophotometerxe2x80x9d by J. U. White et al., Journal of the Optical Society of America, Vol. 47, No. 5, May 1957, pp. 358-376. In later developments, Bouevitch et al. describe, in European Patent Application EP 1126294A2, published Oct. 22, 2001, and in counterpart U.S. Publication No. 2002/0009257A1, the use of diffractive optics in an xe2x80x9cOptical Configuration for a Dynamic Gain Equalizer and a Configurable Add/Drop Multiplexerxe2x80x9d, to modify the signals by such elements as liquid crystal elements, MEMS reflectors, and the like. Emphasis is placed on the 4-f optical system without, however, distinguishing from the substantial earlier work on 4-f spectrometers or fiber coupled spectrometers. Similar to the prior art descriptions, the optical path is folded in the dispersion plane (two dimensional fold) rather than out of the dispersion plane (three dimensional fold). At present, designs and techniques to extend the spectrometer to ultrastable, compact and high performance fiberoptic components have not yet been adequately disclosed.
The invention described herein discloses numerous design features and fabrication approaches specifically tailored for and necessitated by the demanding optical and mechanical requirements of fiberoptic devices. These devices comprise a family of components including dynamic channel equalizers, gain equalizers, band splitters, interleavers, and dynamic add/drop multiplexers. A multiport, wavelength selective, multi-channel variable attenuator and blocker component is the basis of this family of devices. The key features of this component include a unique three dimensional folded optical design, an imaging configuration which improves on the 4-f configuration, optimized anamorphic optics for polarization management, compactness and stability, high performance liquid crystal spatial light modulators (LC-SLMs), and precision alignment processes.
A compact, high optical efficiency system and method for manipulating multi-wavelength optical wavelength signals is based upon three-dimensional refolding, high resolution imaging and modification of wavelength dispersed beams within a compact volume with high optical efficiency. Tracing the beam through the optical system, the DWDM optical signal is first transformed from the fiber mode into an anamorphically shaped, free space propagating beam by a novel micro-optic system. The beams are reflected at slightly different elevations off opposing and spaced apart reflecting and diffractive surfaces so as to converge and focus to asymmetrically shaped, sagittally dispersed individual wavelength components which are separated and projected at the plane of an array of control elements. These control elements may be either dynamic or static, may reflect or transmit, or both, wherein the modified individual wavelength signals are subsequently rediffracted and reimaged into the symmetrical fiber modes for re-launch into the same or another fiber(s). The beam refolding paths may be reversely directed through the same reflecting and diffractive surfaces or through a set of adjacent elements. For DWDM channel spacings of the order of 25 to 50 GHz, the, system provides a very compact, optically efficient and versatile approach which is adaptable to many different applications, and which are optimized to achieve high performance optical specifications.
A combination of high wavelength resolution, compactness and low loss can be attained by employing a diffraction grating in the Littrow configuration, with the grating rulings transverse to the long axis of the anamorphic beam, and a Mangin reflector system for collimating and refolding the beams, and later converging and imaging the wavelength beams onto spatially separate locations. The diffraction grating is angled at the nominal Littrow angle to reflect and disperse individual wavelength components within a sagittal plane, symmetrically distributed about an axis substantially normal to the Mangin mirror system. For 50 GHz and smaller channel spacings, the demands on optical resolution are particularly severe, and require novel high resolution optical designs. In such applications it is preferred to employ a polarization sensitive diffraction grating with high ruling density (e.g. 1100-1200 lines/mm) and a double Mangin mirror system, with four spherical surfaces, of which only the back surface is reflective. This combination is an elegant and practical approach to achieving very flat pass bands, sharp spectral roll-off and low adjacent channel cross-talk, characteristics necessary for the denser channel spacings. With this construction, an 80 to 100 channel DWDM system with 50 GHz channel spacing is realized within a volume which is nominally 15 cm long, less than 10 cm wide and 2 cm tall. 50 GHz optical devices demand a highly refined optical system. However, when channel spacings are greater, e.g. 100 GHz, more design options are feasible, and a polarization independent grating of 600 lpm and a single Mangin reflector may be suitable.
In general, more than one input beam can be launched into the optical system, sharing the reflecting and focusing surfaces while impinging the surfaces at slightly different levels. The polarization components are also separated and refolded in parallel paths to accept more than one beam.
The signal modulators or modifiers for the individual spatially dispersed wavelength components can be used in different system configurations to provide channel equalization, channel blocking, channel add/drop, band splitting and interleaving, or channel switching. A particular example of a channel equalizer/blocker in accordance with the invention uses the reciprocal path configuration for refolding the beams before and after individual control, and an array of reflective liquid crystal elements at the image plane. After phase retardation or variable rotation by the reflective liquid crystal cells, as determined by control signals applied to the cells, the polarization sensitive elements in the signal path block or divert that portion of the signal to be rejected. The wavelength signal components are variably attenuated for channel equalization purposes, or fully extinguished ( greater than 40 dB) for block purposes.
In accordance with another aspect of the invention, the polarization splitter comprises a Wollaston prism combination close to the input anamorpbic collimator device, to separate the orthogonal s and p polarization components into separate beams diverging at a small angle, such that the two beams coincide and overlap at the plane of the modulator. This overlap at the LC-SLM, for example, ensures that both polarization components experience the same amount of retardation which is necessary to give zero PDL and PDF (polarization dependent loss and polarization dependent frequency) by design. Both polarization components are also polarized parallel to one another and parallel to the grating rulings. In their return paths from the reflective liquid crystal elements, the upper beam is directed along the lower beam path and visa versa. Following recombination upon the second pass of the grating, the wavelength components are collapsed back into two orthogonally polarized beams, which are then reunified by the polarization splitter into a single beam for return to the optical fiber mode.
For fixed filtering applications, a specifically patterned template may be disposed at the image plane, to reflect or transmit individual channel signals. The system may be single sided, reflecting signals reciprocally back along the same optics to a common input/output port, or to a separate output port slightly displaced from the input. The system can also be double sided, transmitting channel signals through the template and into a second complementary diffractive Fourier optics assembly. Both optical assemblies can be used at the same time, increasing the combinations that are possible with a static system. Furthermore, two different level gratings and/or two different ports can be used in each beam refolding combination, so that either port can be used as either input or output. With such flexibility the static multichannel template can be used to provide selective channel blocking, drop and throughput patterns, 1xc3x972 channel splitting, and interleaver operation.
In another system in accordance with the invention the wavelength signal modifier array may comprise static or dynamic elements which shift the elevation level of one or more channel input signals or block of signals, for direction to one or more different level output ports. For an interleaver, DWDM signals from one input port can be divided into odd and even channels combined at separate output ports. This takes advantage of the fact that multiple low profile beam patterns at different levels can be refolded concurrently within the same compact volume.
For high efficiency reflection, adequate response times and simplicity of operation, it is preferred to employ zero twist nematic liquid crystal cells, which function as half wave retardation elements for full extinction, and as variable retardation elements for analog control of attenuation. The dispersion of the cells in the sagittal plane may be chirped to precisely map the dispersion plane to the ITU wavelength standard. Other types of spatial light modulators, such as MEMs and twisted nematic liquid crystal cells may be used for dynamic signal control depending upon the application. High density microlithographic patterns may be used to define the multi-channel controls for static blocking, attenuation, reflection or transmission of signals in different channels.
Various tuning and alignment techniques can be used to reduce insertion loss and reduce optical aberrations. For example, the ambient pressure and/or environment within the enclosed housing may be used to achieve period and phase tuning, and the positions and angles of various optical elements are maintained within close limits to maintain beam uniformity, low PDL and low PDF.
Associated optical circuits and devices can be employed to provide other capabilities for meeting multi-channel requirements of DWDM systems, based on the use of diffractive Fourier optics modules, which may employ one or more fiberoptic ports. Combinations of fiberoptic splitters, circulators and thin film filters may be used to enable unique system combinations for achieving add/drop multiplexing, demultiplexing, and equalization.